The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the invention might also find application in conjunction with other diagnostic devices in which equipment is rotated peripherally around an examination region.
In CT scanners, a fan shaped beam of radiation is rotated around a portion of a patient disposed in a region of interest or scan circle. Radiation detectors disposed across the scan circle receive rays of radiation that have traversed the scan circle and the patient. The detectors are periodically sampled to measure the intensity of radiation impinging thereon. Concurrently with each radiation intensity sampling, the position of the x-ray source is monitored. The positions of the x-ray source and the sampled detector describe the path or ray of radiation with which the intensity reading is attributed. Various image reconstruction algorithms may be implemented to reconstruct an image representation from the intensity samplings and the x-ray source position readings. Thus, the accuracy with which the position of the x-ray source is determined affects the accuracy and resolution of the resultant image.
Various hardware has been developed for measuring the angular position of a rotating gantry on which the x-ray source is mounted. In one technique, a belt was driven by gantry rotation to drive an incremental position encoder. In another technique, the incremental encoder was driven by a direct gear drive rather than a belt drive. The position encoder produced an output stream of pulses whose frequency was indicative of the speed of rotation of the gantry. That is, a pulse was generated each time the gantry rotated a preselected increment of arc.
A typical incremental position encoder included a light beam detected by an photodiode or other optical detector. The belt drive rotated a slotted wheel which broke and passed the light beam as the gantry rotated. Magnetic incremental encoders have also been used to indicate rotation. Hall effect sensors have been used to monitor breaking and passing of magnetic flux lines such as with a slotted ferrous wheel or moving magnetic sources. Magnetic pickups have been used as ferrous material proximity detectors to produce electrical pulses in response to passing ferrous teeth or vanes. To determine the direction of rotation, two analogous encoders provided a pair of pulse trains with a preselected phase relationship other than 0.degree. or 180.degree., e.g. a 90.degree. or quadrature relationship. Direction of rotation was indicated by which pulse train was leading and which was trailing.
One of the problems with belt drives is that the belts tend to stretch, adversely affecting the accuracy of the encoder output. Also, there tends to be play or slippage of the belt relative to the drive and driven gears. In a direct gear drive, there is play between the gears to keep them from binding. As the gears wear, the play becomes greater.
One solution for eliminating the play of belt and gear drives is described in U.S. Pat. No. 4,015,129. A hollow glass timing disc etched with a periodic grating circumscribed the scan circle. An optical sensor, such as a light source and photodetector was mounted on the gantry to shine light through the periodic grating marks on the timing disc. In this manner, rotation of the optical sensor relative to the timing disc caused a series of pulses, each pulse occurring after a preselected angular increment of rotation. In order to reduce positional reading errors attributable to mechanical play, a Teflon rider was provided between the ring and the optical sensor. One of the problems with the hollow glass incremental encoder is that it was very sensitive to scratches and manufacturing blemishes.
The prior art mechanically driven incremental sensors had an inherent inaccuracy due to mechanical play. Moreover, the incremental encoders only provided indications of incremental movement. Additional circuitry was required to establish an initial reference or starting point for the scanner and to determine the current position from the pulse trains.
The present invention provides a new and improved position encoder which overcomes the above referenced problems and others.